This invention relates to transducers in general and more particularly to a circuit which compensates for nonlinearity in a transducer arrangement.
The prior art is replete with a number of circuits which operate to linearize the output of a transducer. Essentially, transducers such as piezoresistive devices will exhibit a varying output voltage upon application to such a device of a varying pressure or force. In regard to transducers in general it is desirable that the output voltage be linearily related to the applied pressure or force and thus exhibit a straight line characteristic.
In practice such devices do not exhibit a straight line characteristic but are nonlinear and for example, the output voltage for a certain range of pressures does not follow the same slope as for a lower range of voltages. This results in a nonlinear transfer characteristic of the transducer.
It is known in the prior art on how to achieve compensation of such a device. Thus the prior art has many examples of circuits which will increase the voltage applied to the transducer as the pressure increases. In this way the nonlinearity is compensated for. However, there is a problem associated with a differential pressure transducer which cannot be accomodated by the prior art techniques.
In a differential pressure transducer there is provided a positive and negative output for a range of differential pressures which are applied to the transducer. For example, a differential transducer provides the output voltage which is the differance between first and second input pressures. As one can ascertain, a first input pressure designated as P.sub.1 and a second input pressure designated as P.sub.2 are compared in a differential transducer and an output voltage is obtained which is a difference between P.sub.1 and P.sub.2. Hence if P.sub.1 is greater than P.sub.2, the output voltage will be positive. If P.sub.2 is greater than P.sub.1, the output voltage will be negative.
In such a device the transfer characteristic is such that by applying the prior art techniques one often may not be able to compensate in both the positive and negative directions. Such a situation occurs when the linearity of the device shows a characteristic as given by following equation: EQU V=k.sub.1 F .+-.k.sub.2 F.sup.2
where
V is the output voltage PA1 F is the applied force or other variable PA1 k.sub.1 and k.sub.2 are the sensitivity constant of the particular transducer.
Such a characteristic is very typical of transducers.
In using conventional techniques one will improve the linearity for one polarity output but the same circuit will degrade the linearity for the opposite polarity output. Thus in a differential transducer the circuit which provides linearity in one direction actually degrades or worstens the results in an opposite direction.
Differential pressure transducers are well-known in the field and basically have two inputs for receiving the input pressures as P.sub.1 and P.sub.2 and an output which provides a voltage proportional to the difference between the input pressures.
In the fabrication of the differential pressure tranducer utilizing semiconductor technique it is known that the transfer function of the device is nonlinear and due to the fact that differential pressure is being monitored, linearity cannot be improved or compensated for by using conventional techniques.
It is therefore an object of the present invention to provide apparatus for linearizing the output of a differential pressure transducer.